Composition for injection molding, sintered compact, and method for producing sintered compact

ABSTRACT

A composition for injection molding includes: an inorganic powder composed of at least one of a metal material and a ceramic material; and a binder containing a polyacetal-based resin and an ethylene-glycidyl methacrylate-based copolymer. In the composition, the ethylene-glycidyl methacrylate-based copolymer is contained in an amount of 1% by mass or more and 30% by mass or less with respect to the amount of the polyacetal-based resin.

BACKGROUND

1. Technical Field

The present invention relates to a composition for injection molding, asintered compact, and a method for producing a sintered compact.

2. Related Art

A powder metallurgy process for producing a metal product by sintering amolded body containing a metal powder has been widely used in manyindustrial fields recently because a near net shape sintered compact canbe obtained using the process. Further, a ceramic powder can be used inplace of a metal powder. There are many methods for producing a moldedbody (molding methods), however, a powder injection molding method inwhich an inorganic powder and an organic binder are mixed and kneaded,and injection molding is performed using the resulting kneaded material(compound) is known. A molded body produced by such a powder injectionmolding method is then subjected to a degreasing treatment to remove theorganic binder, followed by firing, whereby a metal product or a ceramicproduct in a desired shape is obtained.

In such a powder injection molding method, it is necessary to select anappropriate organic binder according to various purposes, for example,for the purpose of imparting shape retainability to the molded body.

For example, JP-A-2008-75153 discloses, as an organic binder to be usedin a powder injection molding method, a polyacetal resin, a polystyrene,a polyolefin, a higher fatty acid, and the like.

However, in the case of molding into a complicated shape, unless theshape retainability of the molded body is sufficiently high whendegreasing, it is difficult to maintain the shape of the molded bodywhen degreasing and sintering as it is immediately after molding, andtherefore, deformation, chipping, or the like occurs which deterioratesthe dimensional accuracy of a sintered compact. Due to this, it isdesirable to further increase the shape retainability when degreasing.

SUMMARY

An advantage of some aspects of the invention is to provide acomposition for injection molding capable of producing a molded bodyhaving high shape retainability when degreasing, and also capable ofproducing a sintered compact which is less deformed, chipped, or thelike and which has high quality, a sintered compact having highdimensional accuracy produced using such a composition for injectionmolding, and a method for producing a sintered compact capable ofefficiently producing such a sintered compact.

An aspect of the invention is directed to a composition for injectionmolding including an inorganic powder composed of at least one of ametal material and a ceramic material and a binder containing apolyacetal-based resin and an ethylene-glycidyl methacrylate-basedcopolymer, wherein the ethylene-glycidyl methacrylate-based copolymer iscontained in an amount of 1% by mass or more and 30% by mass or lesswith respect to the amount of the polyacetal-based resin.

According to this configuration, a composition for injection moldingcapable of producing a molded body having high shape retainability whendegreasing is obtained. As a result, a composition for injection moldingcapable of producing a sintered compact which is less deformed, chipped,or the like and which has high quality is obtained.

It is preferred that the composition has an inner layer, which iscomposed mainly of the ethylene-glycidyl methacrylate-based copolymerand is provided so as to cover the surface of each particle of theinorganic powder, and an outer layer, which is composed mainly of thepolyacetal-based resin and is located outside the inner layer.

According to this configuration, the shape retainability and themoldability can both be achieved.

It is preferred that the polyacetal-based resin is a copolymer offormaldehyde and a comonomer other than formaldehyde. According to thisconfiguration, the shape retainability can be further enhanced.

It is preferred that the ethylene-glycidyl methacrylate-based copolymercontains at least one of vinyl acetate and methyl acrylate as a monomerconstituting the copolymer.

According to this configuration, monomers that more reliably bindinorganic powder particles to one another are used, and therefore, theshape retainability can be particularly enhanced.

It is preferred that the softening point of the ethylene-glycidylmethacrylate-based copolymer is 65° C. or higher and 105° C. or lower.

According to this configuration, the shape retainability and themoldability can both be enhanced.

It is preferred that the composition for injection molding includes asaturated fatty acid.

According to this configuration, the saturated fatty acid functions as alubricant, and the moldability of the composition for injection moldingcan be further enhanced. It is preferred that the composition forinjection molding includes a wax.

According to this configuration, the fluidity of the composition forinjection molding is increased, and the moldability is further enhanced.Further, when degreasing, the ethylene-glycidyl methacrylate-basedcopolymer is decomposed in a lower temperature range than thepolyacetal-based resin, and therefore, as the temperature is raised, theethylene-glycidyl methacrylate-based copolymer and the polyacetal-basedresin are sequentially discharged, and as a result, the shaperetainability of the molded body when degreasing can be furtherenhanced.

Another aspect of the invention is directed to a sintered compact whichis produced using the composition for injection molding according to theaspect of the invention described above.

According to this configuration, a sintered compact having highdimensional accuracy is obtained.

Still another aspect of the invention is directed to a method forproducing a sintered compact, including: kneading an inorganic powdercomposed of at least one of a metal material and a ceramic material anda binder containing a polyacetal-based resin and an ethylene-glycidylmethacrylate-based copolymer at a temperature between the softeningpoint of the polyacetal-based resin and a temperature 10° C. lower thanthe softening point of the polyacetal-based resin to obtain a kneadedmaterial; molding the kneaded material to obtain a molded body; anddegreasing the molded body, followed by firing, to obtain a sinteredcompact. According to this configuration, a sintered compact having highdimensional accuracy can be efficiently produced.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a cross-sectional view schematically showing a structure of acomposition for injection molding according to an embodiment of theinvention.

FIG. 2 is a cross-sectional view schematically showing a structure of acomposition for injection molding according to an embodiment of theinvention when kneading.

FIGS. 3A and 3B are observed images of compositions for injectionmolding (kneaded materials) of Example 4 and Comparative Example 2,respectively.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, a composition for injection molding, a sintered compact,and a method for producing a sintered compact according to an embodimentof the invention will be more specifically described.

Composition for Injection Molding

The composition for injection molding of the embodiment of the inventionincludes an inorganic powder and a binder and is obtained by kneadingthese components.

The inorganic powder is composed of at least one of a metal material anda ceramic material.

The binder is a component for binding inorganic powder particles to oneanother, and contains a polyacetal-based resin as a component A and anethylene-glycidyl methacrylate-based copolymer as a component B.

By the injection molding of such a composition for injection molding, amolded body which has high shape retainability when degreasing isobtained. Therefore, by firing this molded body, a sintered compactwhich is less deformed, chipped, or the like and which has high qualityis obtained.

Hereinafter, the respective components of the composition for injectionmolding of the embodiment of the invention will be described in detail.

Inorganic Powder

As the inorganic powder, as described above, a powder composed of atleast one of a metal material and a ceramic material is used.Specifically, other than a metal powder and a ceramic powder, a powderof a composite material of a metal material and a ceramic material, anda mixed powder of a metal powder and a ceramic powder can beexemplified.

Examples of the metal material include Mg, Al, Ti, V, Cr, Mn, Fe, Co,Ni, Cu, Zn, Y, Zr, Nb, Mo, Pd, Ag, In, Sn, Ta, W, an alloy of some ofthese metallic elements, and an alloy of any of these metallic elementswith another metallic element, and among these metal materials, onemetal material or a mixture of two or more metal materials may be used.

Among these metal materials, particularly, a stainless steel, a diesteel, a high-speed tool steel, a low-carbon steel, any of a variety ofFe-based alloys such as an Fe—Ni-based alloy, an Fe—Si-based alloy, anFe—Co-based alloy, and an Fe—Ni—Co-based alloy, an Al-based alloy, aTi-based alloy, or the like is preferably used. Such a metal materialhas excellent mechanical properties, and therefore, a sintered compactwhich has excellent mechanical properties and can be used in a widerange of applications is obtained.

Examples of the stainless steel include SUS304, SUS316, SUS317, SUS329,SUS410, SUS430, SUS440, and SUS630.

Examples of the Al-based alloy include an aluminum simple substance andduralumin.

As the Ti-based alloy, a titanium simple substance or an alloy oftitanium and a metallic element such as aluminum, vanadium, niobium,zirconium, tantalum, or molybdenum can be exemplified. Specific examplesthereof include Ti-6Al-4V and Ti-6Al-7Nb. The Ti-based alloy may includea non-metallic element such as boron, carbon, nitrogen, oxygen, orsilicon other than these metallic elements.

Such a metal powder may be produced by any method, but it is possible touse a metal powder produced by an atomization method (a wateratomization method, a gas atomization method, a high-speed rotatingwater stream atomization method, and the like), a reduction method, acarbonyl method, a grinding method, or the like.

Among these metal powders, a metal powder produced by an atomizationmethod is preferably used. According to an atomization method, it ispossible to efficiently produce a metal powder having an extremely smallaverage particle diameter as described above. In addition, it ispossible to obtain a metal powder in which a variation in particlediameter is small, and the particle diameter is uniform. Therefore, whensuch a metal powder is used, it is possible to reliably prevent thegeneration of pores in a sintered compact, and it is thereby possible toimprove the density.

In addition, a metal powder produced by an atomization method has aspherical shape relatively close to a perfect sphere, whereby the metalpowder has excellent dispersibility and fluidity with respect to thebinder. Therefore, it is possible to increase a filling property whenfilling a granulated powder into a mold, and eventually, it is possibleto obtain a denser sintered compact.

Examples of the ceramic material include oxide-based ceramic materialssuch as alumina, magnesia, beryllia, zirconia, yttria, forsterite,steatite, wollastonite, mullite, cordierite, ferrite, sialon, and ceriumoxide; and non-oxide-based ceramic materials such as silicon nitride,aluminum nitride, boron nitride, titanium nitride, silicon carbide,boron carbide, titanium carbide, and tungsten carbide, among theseceramic materials, one ceramic material or a mixture of two or moreceramic materials is used.

The average particle diameter of the inorganic powder to be used in theembodiment of the invention is preferably 1 μm or more and 30 μm orless, more preferably 3 μm or more and 20 μm or less, and further morepreferably 3 μm or more and 10 μm or less. If the inorganic powder hasan average particle diameter within the above range, it is possible toeventually produce a sufficiently dense sintered compact while avoidingsignificant aggregation or a decrease in compressibility when molding.

If the average particle diameter is less than the above lower limit, theinorganic powder is liable to aggregate, and the compressibility whenmolding may be significantly deteriorated. On the other hand, if theaverage particle diameter exceeds the above upper limit, an interspacebetween powder particles is increased in size too much, and thedensification of the finally obtained sintered compact may beinsufficient.

The average particle diameter is obtained by a laser diffraction methodas a particle diameter when the cumulative amount of a powder on avolume basis reaches 50%.

In the case where the inorganic powder to be used in the embodiment ofthe invention is composed of an Fe-based alloy, the tap density thereofis preferably 3.5 g/cm³ or more, more preferably 3.8 g/cm³ or more. Ifthe inorganic powder has a high tap density as described above, thefilling property in the interspace between the particles when molding isparticularly enhanced. Due to this, it is possible to eventually obtaina particularly dense sintered compact. The tap density of the inorganicpowder can be measured according to, for example, the method formeasuring a tap density specified in JIS Z 2512.

The specific surface area of the inorganic powder to be used in theembodiment of the invention is not particularly limited, but ispreferably 0.15 m²/g or more and 0.8 m²/g or less, more preferably 0.2m²/g or more and 0.7 m²/g or less, and further more preferably 0.3 m²/gor more and 0.6 m²/g or less. If the inorganic powder has a largespecific surface area as described above, the surface activity (surfaceenergy) is increased, and therefore, sintering can be achieved even byapplying less energy. Therefore, when a molded body is sintered, thesintering can be achieved in a shorter time, and the shape retainabilityis enhanced. On the other hand, if the specific surface area exceeds theabove upper limit, a contact area between the inorganic powder and thebinder is increased more than necessary, and the stability and fluidityof the composition for injection molding may be deteriorated. Thespecific surface area of the inorganic powder can be measured accordingto, for example, the method for measuring a specific surface area of apowder (solid) by gas adsorption specified in JIS Z 8830.

Binder

The binder to be used in the embodiment of the invention contains, asdescribed above, a polyacetal-based resin as a component A and anethylene-glycidyl methacrylate-based copolymer as a component B.Hereinafter, the respective components will be described in detail.

Component A

The component A is a polyacetal-based resin. The polyacetal-based resinis a polymer having an oxymethylene structure as a unit structure andmay be a homopolymer containing only formaldehyde as a monomer, acopolymer containing formaldehyde and a monomer other than formaldehyde,or the like. However, from the viewpoint of enhancement of shaperetainability, a copolymer is preferably used. Examples of the monomer(comonomer) other than formaldehyde in the copolymer includeoxyalkylenes such as oxyethylene and oxypropylene, and also includeepichlorohydrin, 1,3-dioxolane, diethylene glycol formal, 1,4-butanediolformal, and 1,3-dioxane, and particularly a monomer having anoxyalkylene unit having 2 or more carbon atoms per molecule ispreferably used. The copolymerization amount of the comonomer is notparticularly limited, but is preferably 1 part by mole or more and 10parts by mole or less, more preferably 1 part by mole or more and 6parts by mole or less with respect to 100 parts by mole of the mainmonomer. The monomer sequence in such a copolymer is not particularlylimited, and any of random copolymerization, alternatingcopolymerization, block copolymerization, and graft copolymerization maybe used. As such a polyacetal-based resin, for example, Delrinmanufactured by Du Pont, Inc., Duracon manufactured by Polyplastics Co.,Ltd., Tenac manufactured by Asahi Kasei Chemicals Corporation, Iupitalmanufactured by Mitsubishi Engineering-Plastics Corporation, PolypencoAcetal manufactured by Quadrant Polypenco Japan Ltd., Amilusmanufactured by Toray Industries, Inc., or the like can be used.Further, the component A has a tensile strength of preferably about 30MPa or more and 90 MPa or less, and more preferably about 40 MPa or moreand 80 MPa or less. If the component A has a tensile strength within theabove range, the shape retainability of the molded body after moldingcan be particularly enhanced.

Component B

The component B is an ethylene-glycidyl methacrylate-based copolymer.The ethylene-glycidyl methacrylate-based copolymer is a copolymer havingan ethylene structure and a glycidyl methacrylate structure as unitstructures. Further, the component B may contain, as another comonomer,one or more structures of carboxylic acid vinyl esters such as vinylacetate, vinyl propionate, methyl acrylate, ethyl acrylate, butylacrylate, methyl methacrylate, ethyl methacrylate, and butylmethacrylate; and ethylenically unsaturated ester compounds such as anα,β-unsaturated carboxylic acid alkyl ester. Among these, particularly,the component B preferably contains at least one of vinyl acetate andmethyl acrylate. According to this, the composition for injectionmolding capable of molding a molded body having particularly high shaperetainability is obtained, and it is possible to eventually produce asintered compact which is particularly less deformed, chipped, or thelike and has high quality.

Here, as described above, the composition for injection molding of theembodiment of the invention includes a binder containing apolyacetal-based resin as the component A and an ethylene-glycidylmethacrylate-based copolymer as the component B.

The present inventors made intensive studies of the cause of the lowshape retainability of a composition for injection molding in therelated art when degreasing and the occurrence of a defect such asdeformation or chipping after sintering.

As a result, they found that the cause of the low shape retainability isthat a metallic element in the inorganic powder performs a catalyticfunction to accelerate the decomposition of the binder, whereby thebinder is decomposed more than necessary and the original function ofthe binder to bind the inorganic powder particles to one another isdeteriorated. On the basis of this finding, they found that the aboveproblem can be solved by incorporating both the components A and B as abinder.

Specifically, when the binder contains the above-described twocomponents, the component B flows at the time of kneading the startingmaterials and penetrates to cover the inorganic powder particles. Thisis because since the softening point of the component B is relativelylower than that of the component A, the component B is transformed intoa liquid prior to the component A so that the component B can penetrateinto an interspace between the inorganic powder particles and thebinder. Moreover, since the component B when softening has relativelyhigh fluidity after softening, the component B can efficiently penetratealso into a small space by a capillary phenomenon. As a result, in thekneaded material (the composition for injection molding) obtained bykneading the starting materials, the component B is present so as tocover the inorganic powder particles, and the component A is present soas to cover the outer side thereof (e.g., each inorganic powder particleis encased by the component B, and the component B encasing inorganicpowder particles are within the component A). Further, the significantdecomposition of the component A is suppressed and the component A canbe gradually decomposed in a degreasing step, and therefore, a decreasein shape retainability is suppressed.

FIG. 1 is a cross-sectional view schematically showing a structure ofthe composition for injection molding of the embodiment of theinvention, and FIG. 2 is a cross-sectional view schematically showing astructure of the composition for injection molding of the embodiment ofthe invention when kneading.

As shown in FIG. 1, in a composition for injection molding 10, aplurality of inorganic powder particles 2 are dispersed in a binder 3.In the binder 3, a component A and a component B intermingle with eachother.

When kneading such a composition for injection molding 10, thetemperature of the composition for injection molding 10 is increased byheating from the outside or self-heating accompanying kneading. As aresult, as shown in FIG. 2, in the obtained kneaded material 1, an innerlayer 21 composed mainly of the component B is formed so as to cover thesurface of each particle 2, and an outer layer 22 composed mainly of thecomponent A is located outside the inner layer 21. If such an innerlayer 21 is formed, the inner layer 21 serves as a partition andprevents the contact between the metallic element in the particle 2 andthe outer layer 22, whereby the above-described catalytic function issuppressed. As a result, the rapid decomposition of the binder 3 issuppressed, and a decrease in shape retainability can be avoided.

Since the inner layer 21 is in contact with the particle 2, the innerlayer 21 should have an ability to resist the above-described catalyticfunction, and on the other hand, the outer layer 22 preferably hasexcellent fluidity although the outer layer 22 has a higher softeningpoint than the inner layer 21. In view of this, the present inventorsfound that by using a polyacetal-based resin as the component A and anethylene-glycidyl methacrylate-based copolymer as the component B, bothof the above-described goals can be reliably achieved. By providing theinner layer 21, when a molded body is subjected to a degreasingtreatment, the outer layer 22 is not rapidly decomposed by heat, butinstead is gradually decomposed by heat, and therefore, the shape of themolded body is maintained. Since the ethylene-glycidylmethacrylate-based copolymer has an excellent blocking property againstthe catalytic function of the metallic element and excellentcompatibility with the polyacetal-based resin, the moldability can befurther enhanced. Accordingly, both the shape retainability and themoldability can be achieved. Here, the component B contains a glycidylgroup. The glycidyl group is ring-opened during kneading and molding,and binds to a hydroxy group on the surfaces of the inorganic powderparticles. As a result, high adhesiveness is exhibited between theinorganic powder and the component B, resulting in stably forming theinner layer 21. On the other hand, it is considered that the ethylenestructure provides the above-described blocking property and alsoprovides the compatibility with the component A.

When the component B contains a unit structure derived from theabove-described ethylenically unsaturated ester compound, thisethylenically unsaturated ester compound more reliably binds theparticles 2 to one another, and therefore, the shape retainability canbe particularly enhanced.

The thickness of the inner layer 21 is not particularly limited as longas the inner layer 21 completely covers the surface of the particle 2,however, for example, an average thickness thereof is preferably 1 nm ormore and 2000 nm or less, more preferably 2 nm or more and 1000 nm orless. According to this, both the shape retainability and themoldability (shape transferability) can be highly achieved. If theaverage thickness of the inner layer 21 is lower than the above lowerlimit, the inner layer 21 is likely to be discontinuous, and therefore,the particle 2 and the outer layer 22 may come into contact with eachother. On the other hand, if the average thickness thereof exceeds theabove upper limit, the ratio of the outer layer 22 is relativelydecreased, and therefore, the moldability may be deteriorated.

The outer layer 22 need not be in the form of a layer as long as it islocated outside the inner layer 21, and may be in the form such that theouter layers 22 associated with the respective particles 2 are connectedto one another, i.e., as shown in FIG. 2, in the kneaded material 1, theouter layer 22 may be in the form of a matrix in which the particles 2are dispersed. Similarly, the inner layers 21 may not be independent ofone another with respect to each particle 2, and may be distributed soas to connect the particles 2 to one another.

The inner layer 21 may be mainly composed of the component B, but maycontain the component A or another component. Similarly, the outer layer22 may be mainly composed of the component A, but may contain thecomponent B or another component. The content of the component B in theinner layer 21 may be more than 50% on a mass basis, and similarly, thecontent of the component A in the outer layer 22 may be more than 50% ona mass basis.

At a boundary between the inner layer 21 and the outer layer 22, theconstituent materials may continuously change through the interface,however, it is preferred that the constituent materials discontinuouslychange. According to such a configuration, the interface between theinner layer 21 and the outer layer 22 serves as a sliding surface, andthe fluidity of the composition for injection molding is particularlyimproved. As a result, the moldability at the time of injection moldingis particularly enhanced, and eventually, a sintered compact having highdimensional accuracy is obtained.

The amount of the component B in the composition for injection moldingis set to 1% by mass or more and 30% by mass or less, and preferably 2%by mass or more and 20% by mass or less with respect to the amount ofthe component A. By setting the amount of the component B within theabove range, both the shape retainability and the moldability can behighly achieved. The softening point of the component B is preferably65° C. or higher and 105° C. or lower, and more preferably 70° C. orhigher and 100° C. or lower. According to this, when kneading or moldingthe composition for injection molding, the component B can be reliablysoftened, whereby the inner layer 21 can be formed. A difference insoftening point between the component B and the component A ispreferably 55° C. or more and 120° C. or less, and more preferably 60°C. or more and 115° C. or less. If the difference in softening pointbetween the component B and the component A is within the above range,both the shape retainability and the moldability can be more highlyachieved. As the unit structures constituting the ethylene-glycidylmethacrylate-based copolymer as the component B, an ethylene structure,a glycidyl methacrylate structure, an ethylenically unsaturated estercompound structure, and the like can be used as described above, and theabundance ratios of these structures are as follows: the ratio of theethylene structure is about 60 to 99.9% by mass, the ratio of theglycidyl methacrylate structure is about 0.1 to 20% by mass, and theratio of the ethylenically unsaturated ester compound structure is about0 to 39.9% by mass.

Particularly, the ratio of the glycidyl methacrylate structure ispreferably 1 part by mass or more and 30 parts by mass or less, and morepreferably 2 parts by mass or more and 25 parts by mass or less withrespect to 100 parts by mass of the ethylene structure. According tothis, a balance between the compatibility with the component Aattributed to the ethylene structure and the affinity for the particle 2attributed to the glycidyl methacrylate structure can be highlyachieved, and therefore, both the shape retainability and themoldability can be particularly enhanced.

The ratio of the ethylenically unsaturated ester compound structure ispreferably 20 parts by mass or more and 80 parts by mass or less, andmore preferably 25 parts by mass or more and 75 parts by mass or lesswith respect to 100 parts by mass of the glycidyl methacrylatestructure.

The melt flow rate of the component B is preferably about 0.5 g/10 minor more and 50 g/10 min or less, and more preferably about 3 g/10 min ormore and 40 g/10 min or less. If the melt flow rate is within the aboverange, the inner layer 21 is reliably formed, and therefore, the shaperetainability of the composition for injection molding of the embodimentof the invention is particularly improved. The melt flow rate can bemeasured at a temperature of 190° C. under a load of 2.16 kg accordingto the method specified in JIS K 6922-2.

The tensile strength of the component B is preferably about 4 MPa ormore and 25 MPa or less, and more preferably about 5 MPa or more and 20MPa or less. According to this, the component B has high fluidity alsowhen softening, and therefore, the inner layer 21 can be more reliablyformed. The weight average molecular weight of the component B isappropriately set in consideration of the melt flow rate as describedabove or the like, however, it is, for example, preferably 10,000 ormore and 400,000 or less, and more preferably 30,000 or more and 300,000or less.

Another Component

The binder to be used in the embodiment of the invention may containanother component.

For example, a wax is preferably used. Examples of the wax includenatural waxes including vegetable waxes such as candelilla wax, carnaubawax, rice wax, Japan wax, and jojoba wax; animal waxes such as beeswax,lanolin, and spermaceti wax; mineral waxes such as montan wax,ozokerite, and ceresin; and petroleum-based waxes such as paraffin wax,microcrystalline wax, and petrolatum; and synthetic waxes includingsynthetic hydrocarbons such as polyethylene wax; modified waxes such asmontan wax derivatives, paraffin wax derivatives, and microcrystallinewax derivatives; hydrogenated waxes such as hydrogenated castor oil andhydrogenated castor oil derivatives; fatty acids such as12-hydroxystearic acid; acid amides such as stearic acid amide; andesters such as phthalic anhydride imide. Among these waxes, one wax canbe used or two or more waxes can be used in combination. Byincorporating such a wax, the fluidity of the composition for injectionmolding is increased, and therefore the moldability thereof is furtherenhanced.

As the wax, particularly a petroleum-based wax or a modified productthereof is preferably used, and paraffin wax, microcrystalline wax,carnauba wax, or a derivative thereof is more preferably used, andparaffin wax or carnauba wax is further more preferably used. Such a waxhas excellent compatibility with the component A, and therefore enablesthe preparation of a homogeneous binder. Accordingly, such a waxcontributes to the production of the composition of injection moldinghaving high moldability.

The weight average molecular weight of the wax is preferably 100 or moreand less than 10,000, and more preferably 200 or more and 5,000 or less.By setting the weight average molecular weight of the wax within theabove range, the inorganic powder and the binder can be more uniformlymixed, and therefore, the moldability of the composition for injectionmolding can be further enhanced.

The content of the wax in the binder is preferably 0.1% by mass or moreand 20% by mass or less, and more preferably 1% by mass or more and 15%by mass or less. By setting the content of the wax within the aboverange, the fluidity of the composition for injection molding can beparticularly increased.

The ratio of the wax to the component B is preferably 0.01 or more and0.5 or less, and more preferably 0.02 or more and 0.4 or less. Bysetting the ratio of the wax to the component B within the above range,a balance between the component B and the wax is optimized, andtherefore, the moldability can be enhanced without impairing the shaperetainability when degreasing.

As the wax, a wax having a softening point of 30° C. or higher and 200°C. or lower is preferably used, and a wax having a softening point of50° C. or higher and 150° C. or lower is more preferably used.

Additional examples of the another component include higher fatty acidssuch as stearic acid, oleic acid, and linoleic acid; higher fatty acidamides such as stearic acid amide, palmitic acid amide, and oleic acidamide; higher alcohols such as stearin alcohol and ethylene glycol;fatty acid esters such as palm oil; phthalic acid esters such as diethylphthalate and dibutyl phathalate; adipic acid esters such as dibutyladipate; sebacic acid esters such as dibutyl sebacate; polyvinylalcohol, polyvinylpyrrolidone, polyether, polypropylene carbonate,ethylenebisstearamide, sodium alginate, agar, gum arabic, resins,sucrose, and ethylene-vinyl acetate copolymers (EVA). Among thesecomponents, one component can be used or two or more components can beused in combination.

Among these, as the higher fatty acid, particularly, a saturated fattyacid such as lauric acid, myristic acid, palmitic acid, stearic acid, orarachidic acid is preferably used. Such a saturated fatty acid containsa long-chain alkyl group, but does not contain an unsaturated bond, andtherefore functions as a lubricant and can further enhance themoldability of the composition for injection molding.

The content of the saturated fatty acid in the binder is preferably 0.1%by mass or more and 10% by mass or less, and more preferably 1% by massor more and 8% by mass or less. By setting the content of the saturatedfatty acid within the above range, the moldability of the compositionfor injection molding can be particularly enhanced.

The ratio of the saturated fatty acid to the component B is preferably0.01 or more and 1 or less, and more preferably 0.02 or more and 0.5 orless. By setting the ratio of the saturated fatty acid to the componentB within the above range, both the shape retainability and themoldability can be more highly achieved.

Further additional examples of the other component include polyolefinssuch as polyethylene, polypropylene, polybutylene, and polypentene;polyolefin-based copolymers such as a polyethylene-polypropylenecopolymer and a polyethylene-polybutylene copolymer; andhydrocarbon-based resins such as polystyrene.

The composition for injection molding may further contain anantioxidant, a degreasing accelerating agent, a surfactant, or the likeother than the above-described components.

The content of the binder in the composition for injection molding isappropriately set according to the metal powder or the ceramic powder,however, it is set to preferably about 1 part by mass or more and 50parts by mass or less, and more preferably about 3 parts by mass or moreand 30 parts by mass or less with respect to 100 parts by mass of theinorganic powder. According to this, the shape retainability of thecomposition for injection molding when degreasing is particularlyenhanced.

The composition for injection molding is prepared by kneading theabove-described inorganic powder, binder, and the like. When kneadingthese components, any of various kneading machines, for example, apressure or double-arm kneader-type kneading machine, a roller-typekneading machine, a Banbury-type kneading machine, a single-screw ortwin-screw extruding machine, or the like can be used.

The kneading conditions may vary depending on various conditions such asthe particle diameter of the metal powder to be used, the mixing ratioof the metal powder to the binder composition, and the like, however,for example, the kneading temperature can be set to 50 to 200° C., andthe kneading time can be set to 15 to 210 minutes.

Method for Producing Sintered Compact

Hereinafter, a method for producing a sintered compact according to anembodiment of the invention will be described. The method for producinga sintered compact includes: a kneading step in which an inorganicpowder and a binder are kneaded, thereby obtaining a kneaded material(the composition for injection molding); a molding step in which thethus obtained kneaded material is molded into a desired shape; adegreasing step in which the thus obtained molded body is degreased; anda firing step in which the thus obtained degreased body is fired.Hereinafter, the respective steps will be sequentially described.

Kneading Step

The kneaded material is prepared by kneading the above-describedinorganic powder, binder, and the like. When kneading these components,any of various kneading machines, for example, a pressure or double-armkneader-type kneading machine, a roller-type kneading machine, aBanbury-type kneading machine, a single-screw or twin-screw extrudingmachine, or the like can be used.

The kneading temperature in the kneading step is preferably setaccording to the softening points of the components A and B.Specifically, since the ethylene-glycidyl methacrylate-based copolymeras the component B has a lower softening point than the polyacetal-basedresin as the component A, the initial kneading temperature is preferablyset to a temperature between the softening point of the component A anda temperature lower than the softening point of the component A by about10° C. By kneading the components at such a temperature, only thecomponent B is softened as the temperature is raised when kneading sothat it becomes easy for the component B to penetrate between theparticle 2 and the component A. As a result, the inner layer 21 and theouter layer 22 are formed, whereby both the shape retainability and themoldability can be highly achieved.

When the softening point of the component A is represented by T_(A)° C.and the softening point of the component B is represented by T_(B)° C.,as described above, the kneading temperature is preferably (T_(A)−10)°C. or higher and T_(A)° C. or lower, more preferably (T_(A)−10)° C. orhigher and (T_(A)−2)° C. or lower. By kneading the components at such atemperature, the above-described effect becomes more pronounced.Further, it is preferred that such a kneading temperature is maintainedfor about 5 minutes or more and 180 minutes or less, with the provisothat T_(A) and T_(B) preferably satisfy the relationship:T_(A)−10>T_(B).

After completing the kneading under the above conditions, kneading maybe performed at a temperature higher than the softening point of thecomponent A (T_(A)) in the end. According to this, also the component Ais softened, and the fluidity of the entire kneaded material is furtherimproved. In this case, the final kneading temperature is preferablyhigher than T_(A)° C. and (T_(A)+70)° C. or lower.

The total kneading time is preferably about 15 minutes or more and 210minutes or less.

The viscosity of the thus obtained kneaded material is preferably 500 Por more and 7,000 P or less (50 Pa·s or more and 700 Pa·s or less), morepreferably 1,000 P or more and 6,000 P or less (100 Pa·s or more and 600Pa·s or less). According to this, the moldability when molding can beparticularly enhanced. The viscosity is measured using a capirograph bymaintaining the temperature of the kneaded material at 190° C. As thebinder to be subjected to this step, a binder in the form of a powder ispreferably used. When transforming the binder into a powder, a commongrinding method is used, however, particularly, cryogenic grinding ispreferably used. A binder powder obtained by cryogenic grinding isparticularly fine and uniform, and moreover, has the original binderproperty since a heating effect when grinding is suppressed. Therefore,the above-described effect on the basis of the difference in softeningpoint between the component A and the component B is more reliablyexhibited. As a result, the inner layer 21 and the outer layer 22 arereliably formed around the circumference of the inorganic powderparticle, whereby a kneaded material which can highly achieve both theshape retainability and the moldability is obtained.

The cryogenic grinding is a method of finely and uniformly grinding asample by utilizing the brittleness caused by the freezing of thesample. In the cryogenic grinding, a cryogenic grinding machine is used.The cryogenic grinding machine is provided with a grinding vessel, inwhich a sample is placed, and steel balls, which reciprocate in thegrinding vessel, and by causing the steel balls to reciprocate whilecooling the grinding vessel with a cooling agent such as liquidnitrogen, a sample in the grinding vessel is ground. As coolingprogresses, the sample becomes brittle, and therefore, a sample withflexibility can also be ground. The above-described cryogenic grindingmachine is described as one example, and a cryogenic grinding machinehaving other structure can also be used.

By subjecting the binder to cryogenic grinding, the binder can be groundfinely and uniformly without denaturing the binder. In the case of usinga grinding method other than cryogenic grinding, heat is generated inthe binder as grinding progresses, and due to this heat, denaturation,melting (softening), or decomposition may occur, however, by usingcryogenic grinding, this denaturation, melting, or decomposition can beprevented. As a result, the binder is to be subjected to the followingstep while maintaining the original property, and therefore, a decreasein shape retainability of a molded body is prevented. Eventually, it ispossible to produce a sintered compact which is less deformed, chipped,or the like and has high quality.

When using cryogenic grinding, the resulting powder is fine and has alarge specific surface area and also has a high surface activity. Such apowder has high affinity for the inorganic powder, and when mixing thebinder powder with the inorganic powder, the powder contributes to thesuppression of the occurrence of a problem such as uneven mixing.Accordingly, the use of cryogenic grinding enables the production of aparticularly uniform composition for injection molding.

As the cooling agent in cryogenic grinding, other than liquid nitrogenas described above, liquid air, liquid oxygen, dry ice, or the like maybe used.

In the case of performing cryogenic grinding, cryogenic grinding may beperformed after mixing the components A and B, or the components A and Bmay be separately subjected to cryogenic grinding, however, from theviewpoint of obtaining a homogeneous kneaded material by uniformlymixing the components A and B, the latter process is preferred.

The thus obtained binder powder has an average particle diameter ofpreferably about 10 μm or more and 500 μm or less, and more preferablyabout 15 μm or more and 400 μm or less. By grinding the binder throughcryogenic grinding to a particle diameter within the above range, theeffect of a difference in specific gravity during mixing can besuppressed to minimum, and therefore, the binder powder and theinorganic powder can be uniformly mixed.

Particularly, in the case of grinding the components A and B separately,the average particle diameter of the binder powder of the component A ispreferably 3 times or more and 20 times or less, and more preferably 7times or more and 15 times or less larger than that of the inorganicpowder. Meanwhile, the average particle diameter of the binder powder ofthe component B is preferably 3 times or more and 50 times or less, andmore preferably 5 times or more and 30 times or less larger than that ofthe inorganic powder. According to this, the binder powder and theinorganic powder can be more uniformly mixed. The average particlediameter of the binder powder of the component B is preferably 2 timesor more and 15 times or less, and more preferably 3 times or more and 10times or less larger than that of the binder powder of the component A.According to this, the binder powder of the component A and the binderpowder of the component B can be more uniformly mixed.

The average particle diameter is obtained by a laser diffraction methodas a particle diameter when the cumulative amount of a powder on avolume basis reaches 50%.

Molding Step

First, the composition for injection molding of the embodiment of theinvention as described above is molded. According to this, a molded bodyhaving a desired shape and dimension is produced.

As the molding method, an injection molding method is used.Incidentally, prior to injection molding, the composition for injectionmolding may be subjected to a pelletization treatment as needed. Thepelletization treatment is a treatment in which a compound is groundusing a grinding device such as a pelletizer. The thus obtained pelletshave an average particle diameter of about 1 mm or more and 10 mm orless. Subsequently, the thus obtained pellets are placed in an injectionmolding machine and injected into a mold to effect molding. According tothis, a molded body to which the shape of the mold has been transferredis obtained.

The shape and dimension of the molded body to be produced are determinedin anticipation of the amount of shrinkage by degreasing and sinteringto be performed thereafter.

The resulting molded body may be subjected to post-processing such asmechanical processing or laser processing as needed. The injectionpressure for the composition for injection molding is preferably fromabout 5 to 500 MPa.

Further, it is preferred that not only the temperature in the moldingstep, but also the temperature when the composition for injectionmolding is kneaded (kneading temperature) is set in the same manner asthe above-described molding temperature. According to this, the innerlayer 21 is formed also at the kneading stage, and therefore, thefluidity of the kneaded material is improved and uniform kneading can beachieved. Accordingly, also the resulting molded body becomeshomogeneous, and the shape retainability and the moldability are furtherimproved.

Degreasing Step

Subsequently, the thus obtained molded body is subjected to a degreasingtreatment. According to this, the binder contained in the molded body isremoved (degreased), whereby a degreased body is obtained.

The component B is decomposed and discharged outside before thecomponent A when degreasing or molding prior to degreasing in manycases. At this time, a flow path is formed in the molded body. In thedegreasing step, a decomposed product of the component A is dischargedthrough this flow path, and therefore, a degreasing treatment can beperformed while preventing the occurrence of a crack or the like in themolded body. As a result, the shape retainability of the molded body(degreased body) can be particularly enhanced.

The degreasing treatment is not particularly limited, but is performedby a heat treatment in an oxidative atmosphere such as oxygen gas ornitric acid gas, and other than this, in a non-oxidative atmosphere, forexample, under vacuum or a reduced pressure (for example, 1.33×10⁻⁴ Paor more and 13.3 Pa or less), or in a gas such as nitrogen gas or argongas. The treatment temperature in the degreasing step (heat treatment)is not particularly limited, but is preferably 100° C. or higher and750° C. or lower, and more preferably 150° C. or higher and 700° C. orlower.

The treatment time (heat treatment time) in the degreasing step (heattreatment) is not particularly limited, but is preferably 0.5 hours ormore and 20 hours or less, and more preferably 1 hour or more and 10hours or less.

The degreasing by such a heat treatment may be performed by beingdivided into a plurality of steps (stages) for various purposes (forexample, for the purpose of reducing the degreasing time, and the like).In this case, for example, a method in which degreasing is performed ata low temperature in the former half and at a high temperature in thelatter half, a method in which degreasing at a low temperature anddegreasing at a high temperature are alternately repeated, or the likecan be used.

After the degreasing treatment as described above, the thus obtaineddegreased body may be subjected to various post-processing treatmentsfor the purpose of, for example, deburring, forming a microstructuresuch as a groove, and the like.

It is not necessary to completely remove the binder in the molded bodyby the degreasing treatment, and for example, the binder may partiallyremain therein at the time of completion of the degreasing treatment.

Firing Step

Subsequent to the degreasing treatment, the degreased body is fired.According to this, the degreased body is sintered, whereby a sinteredcompact (the sintered compact of the embodiment) is obtained.

The firing conditions are not particularly limited, but the firing stepis performed by a heat treatment in a non-oxidative atmosphere, forexample, under vacuum or a reduced pressure (for example, 1.33×10⁻⁴ Paor more and 133 Pa or less), or in an inert gas such as nitrogen gas orargon gas. According to this, the oxidation of the metal powder can beprevented. In the case where a metal material is contained in theinorganic powder, it is preferred that when firing, the degreased bodyis placed in a vessel composed of a metal material of the same type asthe metal material contained in the inorganic powder, and the degreasedbody is fired in such a state. According to this, the metal component inthe degreased body is hardly evaporated, and therefore, the metalcomposition of the finally obtained sintered compact can be preventedfrom deviating from the intended composition.

As the vessel to be used, a vessel not having an airtight structure, butinstead having an appropriate pore or aperture is preferred. Accordingto this, the atmosphere in the inside of the vessel is made the same asthat in the outside of the vessel, and can be prevented from changing toan undesired atmosphere. Further, it is preferred that there is asufficient space between the vessel and the degreased body withoutadhering to each other as much as possible.

The atmosphere in which the firing step is performed may be changed inthe course of the firing step. For example, the initial firingatmosphere is set to a reduced pressure atmosphere, and then, theatmosphere can be changed to an inert atmosphere in the course of thefiring step.

The firing step may be performed by being divided into two or morestages. According to this, sintering efficiency is improved, andsintering can be achieved in a shorter firing time.

It is preferred that the firing step is performed continuously with theabove-described degreasing step. According to this, the degreasing stepcan also serve as a pre-sintering step, and therefore, preheating isprovided for the degreased body and the degreased body can be morereliably sintered.

The firing temperature is appropriately set according to the type of theinorganic powder. However, in the case of the metal powder, the firingtemperature is preferably 1,000° C. or higher and 1,650° C. or lower,and more preferably 1,050° C. or higher and 1,500° C. or lower.Meanwhile in the case of the ceramic powder, the firing temperature ispreferably 1,250° C. or higher and 1,900° C. or lower, and morepreferably 1,300° C. or higher and 1,800° C. or lower.

The firing time is preferably 0.5 hours or more and 20 hours or less,and more preferably 1 hour or more and 15 hours or less.

Such a firing step may be performed by being divided into a plurality ofsteps (stages) for various purposes (for example, for the purpose ofreducing the firing time). In this case, for example, a method in whichfiring is performed at a low temperature in the former half and at ahigh temperature in the latter half, a method in which firing at a lowtemperature and firing at a high temperature are alternately repeated,or the like can be used.

After the firing step as described above, the thus obtained sinteredcompact may be subjected to mechanical processing, electric dischargeprocessing, laser processing, etching, or the like for the purpose of,for example, deburring, forming a microstructure such as a groove, orthe like The obtained sintered compact may be subjected to an HIPtreatment (hot isostatic press treatment) or the like as needed.According to this, the density of the sintered compact can be furtherincreased.

As for the conditions for the HIP treatment, for example, thetemperature is set to 850° C. or higher and 1,100° C. or lower, and thetime is set to 1 hour or more and 10 hours or less. Further, thepressure to be applied is preferably 50 MPa or more, and more preferably100 MPa or more.

The sintered compact obtained as described above may be used in anypurpose, and as the use thereof, various structural parts, variousmedical structures, and the like can be exemplified. The relativedensity of the thus obtained sintered compact is expected to be, forexample, 95% or more, and preferably 96% or more. Such a sinteredcompact has a high sintering density and has excellent appearance anddimensional accuracy. Further, the tensile strength of the sinteredcompact is expected to be, for example, 900 MPa or more in the case ofusing a metal powder. In addition, the 0.2% proof stress of the sinteredcompact is expected to be, for example, 750 MPa or more in the case ofusing a metal powder.

Hereinabove, the invention has been described based on preferredembodiments, however, the invention is not limited thereto.

EXAMPLES

Next, specific Examples will be described.

1. Production of Sintered Compact

Example 1

First, an SUS316L powder (powder No. 1) produced by a water atomizationmethod was prepared. The average particle diameter of the SUS316L powderwas measured using a laser diffraction particle size distributionanalyzer (Microtrac HRA 9320-X100, manufactured by Nikkiso Co., Ltd.).The measured values are shown in Table 1.

TABLE 1 Amount of binder with Average particle respect to 100 parts bydiameter mass of powder Formulation [μm] (parts by mass) Powder No. 1SUS316L 10 10 Powder No. 2 2% Ni-Fe 6 9 Powder No. 3 Ti-6Al-4V 17 11Powder No. 4 Alumina 0.5 30

On the other hand, a binder having a formulation shown in Table 2 wasprepared, and a component A, a component B, and another component suchas a lubricant were separately cryogenically ground. By doing this, afirst binder powder obtained by cryogenically grinding the component A,a second binder powder obtained by cryogenically grinding the componentB, and a third binder powder obtained by cryogenic grinding thelubricant or the like were separately produced.

Specifically, a starting material such as the component A was placed ina grinding vessel and ground while cooling with liquid nitrogen. Thegrinding conditions for the cryogenic grinding were set such that thematerial temperature was −196° C., the grinding machine temperature was−15° C., and the grinding machine rotation speed was 5,200 rpm. Theaverage particle diameters of the obtained first binder powder, secondbinder powder, and third binder powder were 53 μm, 242 μm, and 200 μm,respectively.

Subsequently, the SUS316L powder and the binder powders were mixed andkneaded using a pressure kneader (kneading machine) at a kneadingtemperature of 160° C. for 30 minutes. This kneading was performed in anitrogen atmosphere. The mixing ratio of the SUS316L powder and thebinder is shown in Table 1.

Subsequently, the thus obtained kneaded material was ground using apelletizer, whereby pellets having an average particle diameter of 5 mmwere obtained.

Then, the thus obtained pellets were molded by an injection moldingmachine under the molding conditions that the material temperature was190° C. and the injection pressure was 10.8 MPa (110 kgf/cm²). By doingthis, a molded body was obtained. The molded body had a cylindricalshape with a diameter of 0.5 mm and a height of 0.5 mm.

Subsequently, the molded body was subjected to a degreasing treatmentunder the degreasing conditions that the temperature was 500° C., thetime was 1 hour, and the atmosphere was nitrogen gas (atmosphericpressure). By doing this, a degreased body was obtained.

Subsequently, the degreased body was subjected to a firing treatmentunder the firing conditions that the temperature was 1,270° C., the timewas 3 hours, and the atmosphere was nitrogen gas (atmospheric pressure).By doing this, a sintered compact was obtained.

Examples 2 to 13

Sintered compacts were obtained in the same manner as in Example 1except that a binder having a formulation shown in Table 2 was used asthe binder. Incidentally, in Example 12, the kneading temperature wasset to 155° C.

Comparative Examples 1 to 6

Sintered compacts were obtained in the same manner as in Example 1except that the powder No. 1 was used as the inorganic powder and abinder having a formulation shown in Table 2 was used as the binder.

TABLE 2 Soft- Ex- Ex- Ex- Ex- Ex- Ex- Ex- Ex- Ex- Ex- Classi- eningample ample ample ample ample ample ample ample ample ample ficationComponent point Unit 1 2 3 4 5 6 7 8 9 10 Binder Com- Tenac 170° C. % by97 95 93 88 82 78 74 83 73 88 ponent HC750 mass A Tenac 160° C. % by7520 mass Tenac 165° C. % by 7054 mass Com- E-GMA-  95° C. % by 1 3 5 1015 20 22 8 ponent VA mass B E-GMA-  52° C. % by 12 2 MA mass E-GMA 103°C. % by 17 mass Wax Paraffin  60° C. % by 1 2 3 wax mass Micro-  70° C.% by 1 crystalline mass wax Poly- 110° C. % by 1 ethylene mass wax OtherDibutyl — % by 1 phthalate mass Stearic acid  70° C. % by 2 2 2 2 2 1 32 5 2 mass EVA  45° C. % by 2 mass Polystyrene — % by mass Component B/— % by 1.0 3.2 5.4 11.4 18.3 25.6 29.7 14.5 23.3 11.4 Component A × 100mass Wax/Component B — — 0.00 0.00 0.00 0.00 0.07 0.05 0.05 0.17 0.180.00 Stearic acid/ — — 2.00 0.67 0.40 0.20 0.13 0.05 0.14 0.17 0.29 0.20Component B Inorganic Metal powder — — No. 1 No. 1 No. 1 No. 1 No. 1 No.1 No. 1 No. 1 No. 1 No. 1 powder Kneaded Viscosity — P 3900 3700 34004800 3200 3900 5300 5400 5500 3300 material Evaluation Sintering density— — 97.3 97.8 98.1 98.2 98.2 97.6 96.8 96.7 96.4 98.3 results ofAppearance — — C A A A A A B B B A sintered Dimensional — — B B A A A BB B B A compact accuracy Compar- Compar- Compar- Compar- Compar- Compar-Soft- Ex- Ex- Ex- ative ative ative ative ative ative Classi- eningample ample ample Exam- Exam- Exam- Exam- Exam- Exam- fication Componentpoint Unit 11 12 13 ple 1 ple 2 ple 3 ple 4 ple 5 ple 6 Binder Com-Tenac 170° C. % by 85 98 92 95.5 71 ponent HC750 mass A Tenac 160° C. %by 90 7520 mass Tenac 165° C. % by 85 7054 mass Com- E-GMA-  95° C. % by10 0.5 25 ponent VA mass B E-GMA-  52° C. % by 1 8 3 MA mass E-GMA 103°C. % by 7 mass Wax Paraffin  60° C. % by 1 2 5 2 5 wax mass Micro-  70°C. % by 1 crystalline mass wax Poly- 110° C. % by 1 1 ethylene mass waxOther Dibutyl — % by 1 10 phthalate mass Stearic acid  70° C. % by 2 2 32 2 3 2 3 3 mass EVA  45° C. % by 40 43 mass Polystyrene — % by 55 40mass Component B/ — % by 12.9 8.9 11.8 — — — — 0.5 35.2 Component A ×100 mass Wax/Component B — — 0.18 0.00 0.20 — — — — 2.00 0.04 Stearicacid/ — — 0.18 0.25 0.30 — — — — 6.00 0.12 Component B Inorganic Metalpowder — — No. 1 No. 1 No. 1 No. 1 No. 1 No. 1 No. 1 No. 1 No. 1 powderKneaded Viscosity — P 3400 4100 5000 7500 6900 6400 5900 6600 1700material Evaluation Sintering density — — 98.4 97.8 96.8 96.1 96.4 94.595.7 94.5 95.7 results of Appearance — — A B B D D D D D C sinteredDimensional — — A B C D D D D C D compact accuracy *: E-GMA indicatesthat an ethylene structure and a glycidyl methacrylate structure arecontained. *: VA indicates that a vinyl acetate structure is contained,and MA indicates that a methyl acrylate structure is contained.

The components A and B in Table 2 shown above and Tables 3 to 5 shownbelow are the following compounds.

Component A

-   -   Tenac HC750: a polyacetal-based copolymer    -   Tenac 7520: a polyacetal-based copolymer    -   Tenac 7054: a polyacetal-based homopolymer        Component B    -   E-GMA-VA: a glycidyl methacrylate structure: 12% by mass, a        vinyl acetate structure: 5% by mass, and an ethylene structure:        remainder    -   E-GMA-MA: a glycidyl methacrylate structure: 3% by mass, a        methyl acrylate structure: 27% by mass, and an ethylene        structure: remainder    -   E-GMA: a glycidyl methacrylate structure: 12% by mass and an        ethylene structure: remainder

As for the melt flow rate of the component B, E-GMA-VA, E-GMA-MA, andE-GMA had melt flow rates of 7 g/10 min, 7 g/10 min, and 3 g/10 min,respectively.

Examples 14 to 16

First, a 2% Ni—Fe alloy powder (powder No. 2) produced by a wateratomization method was prepared. The average particle diameter of thepowder was measured using a laser diffraction particle size distributionanalyzer. The measured values are shown in Table 1. The formulation ofthe 2% Ni—Fe alloy is as follows: C (0.4% by mass or more and 0.6% bymass or less), Si (0.35% by mass or less), Mn (0.8% by mass or less), P(0.03% by mass or less), S (0.045% by mass or less), Ni (1.5% by mass ormore and 2.5% by mass or less), Cr (0.2% by mass or less), and Fe(remainder).

Then, sintered compacts were obtained in the same manner as in Example 1except that a binder having a formulation shown in Table 3 was used asthe binder. Incidentally, the kneading conditions were set such that thetemperature was 160° C. and the time was 30 minutes. Further, themolding conditions were set such that the material temperature was 190°C. Further, the degreasing conditions were set such that the temperaturewas 600° C., the time was 1 hour, and the atmosphere was nitrogen gas(atmospheric pressure). Further, the firing conditions were set suchthat the temperature was 1,150° C., the time was 3 hours, and theatmosphere was nitrogen gas (atmospheric pressure).

Comparative Examples 7 to 11

Sintered compacts were obtained in the same manner as in Example 1except that the powder No. 2 was used as the inorganic powder and abinder having a formulation shown in Table 3 was used as the binder.

TABLE 3 Compar- Compar- Compar- Compar- Compar- Ex- Ex- Ex- ative ativeative ative ative Softening ample ample ample Exam- Exam- Exam- Exam-Exam- Classification Component point Unit 14 15 16 ple 7 ple 8 ple 9 ple10 ple 11 Binder Component A Tenac HC750 170° C. % by mass 88 87 96 9296.5 Tenac 7520 160° C. % by mass Tenac 7054 165° C. % by mass 83 72Component B E-GMA-VA  95° C. % by mass 10 7 9 0.5 E-GMA-MA  52° C. % bymass 2 E-GMA 103° C. % by mass Wax Paraffin wax  60° C. % by mass 1 3 52 1 2 Microcrystalline  70° C. % by mass 0.5 wax Polyethylene 110° C. %by mass wax Other Dibutyl phthalate — % by mass 0.5 1 Stearic acid  70°C. % by mass 2 2 5 2 2 3 2 3 EVA  45° C. % by mass 40 Polystyrene — % bymass 55 Component B/Component A × — % by mass 11.4 10.3 10.8 — — — 0.531.9 100 Wax/Component B — — 0.00 0.17 0.33 — — — 2.00 0.09 Stearicacid/Component B — — 0.20 0.22 0.56 — — — 4.00 0.13 Inorganic Metalpowder — — No. 2 No. 2 No. 2 No. 2 No. 2 No. 2 No. 2 No. 2 powderKneaded Viscosity — P 3100 3300 4700 7700 7000 6600 4900 1600 materialEvaluation Sintering density — — 98.1 98.6 97.0 95.9 96.4 94.1 95.3 94.1results of Appearance — — A A B D D D D C sintered Dimensional accuracy— — A A B D D D C D compact *: E-GMA indicates that an ethylenestructure and a glycidyl methacylate structure are contained *: VAindicates that a vinyl acetate structure is contained, and MA indicatesthat a methyl acrylate structure is contained.

Examples 17 to 19

First, a Ti alloy powder (powder No. 3) produced by a gas atomizationmethod was prepared. The average particle diameter of the powder wasmeasured using a laser diffraction particle size distribution analyzer.The measured values are shown in Table 1.

Then, sintered compacts were obtained in the same manner as in Example 1except that a binder having a formulation shown in Table 4 was used asthe binder. Incidentally, the kneading conditions were set such that thetemperature was 160° C. and the time was 30 minutes. Further, themolding conditions were set such that the material temperature was 190°C. Further, the degreasing conditions were set such that the temperaturewas 450° C., the time was 1 hour, and the atmosphere was nitrogen gas(atmospheric pressure). Further, the firing conditions were set suchthat the temperature was 1,100° C., the time was 3 hours, and theatmosphere was argon gas (reduced pressure: 1.3 kPa).

Comparative Examples 12 to 16

Sintered compacts were obtained in the same manner as in Example 1except that the powder No. 3 was used as the inorganic powder and abinder having a formulation shown in Table 4 was used as the binder.

TABLE 4 Compar- Compar- Compar- Compar- Compar- Ex- Ex- Ex- ative ativeative ative ative Softening ample ample ample Exam- Exam- Exam- Exam-Exam- Classification Component point Unit 17 18 19 ple 12 ple 13 ple 14ple 15 ple 16 Binder Component A Tenac HC750 170° C. % by mass 88 87 9892 96.5 Tenac 7520 160° C. % by mass Tenac 7054 165° C. % by mass 89 72Component B E-GMA-VA  95° C. % by mass 10 8 7 0.5 E-GMA-MA  52° C. % bymass E-GMA 103° C. % by mass 2 Wax Paraffin wax  60° C. % by mass 1 5 21 2 Microcrystalline  70° C. % by mass 0.5 3 wax Polyethylene 110° C. %by mass wax Other Dibutyl phthalate — % by mass 0.5 1 Stearic acid  70°C. % by mass 2 1 1 2 2 3 2 3 EVA  45° C. % by mass 40 Polystyrene — % bymass 55 Component B/Component A × — % by mass 11.4 11.5 7.9 — — — 0.531.9 100 Wax/Component B — — 0.00 0.15 0.43 — — — 2.00 0.09 Stearicacid/Component B — — 0.20 0.10 0.14 — — — 4.00 0.13 Inorganic Metalpowder — — No. 3 No. 3 No. 3 No. 3 No. 3 No. 3 No. 3 No. 3 powderKneaded Viscosity — P 3200 3500 4800 7400 7200 6700 5100 6500 materialEvaluation Sintering density — — 97.8 98.3 97.2 95.6 96.1 93.8 95.8 94.2results of Appearance — — A A B D D D D C sintered Dimensional accuracy— — A A B D D D C D compact *: E-GMA indicates that an ethylenestructure and a glycidyl methacylate structure are contained. *: VAindicates that a vinyl acetate structure is contained, and MA indicatesthat a methyl acrylate structure is contained.

Examples 20 to 22

First, an alumina powder (powder No. 4) was prepared, and the averageparticle diameter of the powder was measured using a laser diffractionparticle size distribution analyzer. The measured values are shown inTable 1.

Then, sintered compacts were obtained in the same manner as in Example 1except that a binder having a formulation shown in Table 5 was used asthe binder. Incidentally, the kneading conditions were set such that thetemperature was 160° C. and the time was 30 minutes. Further, themolding conditions were set such that the material temperature was 190°C. Further, the degreasing conditions were set such that the temperaturewas 500° C., the time was 2 hours, and the atmosphere was nitrogen gas(atmospheric pressure). Further, the firing conditions were set suchthat the temperature was 1,600° C., the time was 3 hours, and theatmosphere was air.

Comparative Examples 17 to 21

Sintered compacts were obtained in the same manner as in Example 1except that the powder No. 4 was used as the inorganic powder and abinder having a formulation shown in Table 5 was used as the binder.

TABLE 5 Compar- Compar- Compar- Compar- Compar- Ex- Ex- Ex- ative ativeative ative ative Softening ample ample ample Exam- Exam- Exam- Exam-Exam- Classification Component point Unit 20 21 22 ple 17 ple 18 ple 19ple 20 ple 21 Binder Component A Tenac HC750 170° C. % by mass 88 85 9892 96.5 Tenac 7520 160° C. % by mass Tenac 7054 165° C. % by mass 85 72Component B E-GMA-VA  95° C. % by mass 10 10 0.5 E-GMA-MA  52° C. % bymass 1 3 E-GMA 103° C. % by mass 7 23 Wax Paraffin wax  60° C. % by mass1 2 5 2 1 2 Microcrystalline  70° C. % by mass 1 wax Polyethylene 110°C. % by mass 1 wax Other Dibutyl phthalate — % by mass 0.5 Stearic acid 70° C. % by mass 2 1.5 3 2 2 3 2 3 EVA  45° C. % by mass 40 Polystyrene— % by mass 55 Component B/Component A × — % by mass 11.4 12.9 11.8 — —— 0.5 31.9 100 Wax/Component B — — 0.00 0.18 0.20 — — — 2.00 0.09Stearic acid/Component B — — 0.20 0.14 0.30 — — — 4.00 0.13 InorganicCeramic powder — — No. 4 No. 4 No. 4 No. 4 No. 4 No. 4 No. 4 No. 4powder Kneaded Viscosity — P 3500 3700 4700 7800 7600 8000 7200 1300material Evaluation Sintering density — — 97.2 97.5 96.8 93.1 93.4 92.393.5 93.8 results of Appearance — — A A B D D D D C sintered Dimensionalaccuracy — — A A B D D D C D compact *: E-GMA indicates that an ethylenestructure and a glycidyl methacylate structure are contained. *: VAindicates that a vinyl acetate structure is contained, and MA indicatesthat a methyl acrylate structure is contained.2. Evaluation of Kneaded Material2.1 Evaluation of Viscosity of Kneaded Material

Each of the kneaded materials obtained in Examples and ComparativeExamples was maintained at a temperature of 190° C., and the viscositythereof was measured using a capirograph. The measurement results areshown in Tables 2 to 5.

2.2 Evaluation by Microscopic Observation

Each of the kneaded materials obtained in Examples and ComparativeExamples was placed in fuming nitric acid at 120° C. for 3 hours,whereby the component A was selectively removed from the kneadedmaterial. The polyacetal-based resin as the component A is decomposed ata temperature lower than the softening point in fuming nitric acid, andtherefore can be selectively removed. Accordingly, by performing thistreatment, the outer layer 22 can be selectively removed from thekneaded material. As a result, in the kneaded material, the inorganicpowder and the inner layer 21 mainly remain. Then, the kneaded materialsubjected to the fuming nitric acid treatment was observed by a scanningelectron microscope. In FIGS. 3A and 3B, observed images of the kneadedmaterials obtained in Example 4 and Comparative Example 2 are shown asrepresentatives, respectively.

As shown in FIG. 3A, in the case of the kneaded material obtained inExample 4 subjected to the fuming nitric acid treatment, a state inwhich the inner layer 21 is present so as to connect the inorganicpowder particles to one another is observed. Further, it is observedthat the surface of a substance which looks like a particle hasrelatively high smoothness. Accordingly, it is confirmed that theinorganic powder particles shown in FIG. 3A are covered with the innerlayer 21 without any uncovered areas.

On the other hand, as shown in FIG. 3B, in the case of the kneadedmaterial obtained in Comparative Example 2 subjected to the fumingnitric acid treatment, the inner layer 21 which is present so as toconnect the inorganic powder particles to one another is almost notobserved. Further, it is observed that on the surface of a substancewhich looks like a particle, a difference between light and shade islarge, and the surface has relatively low smoothness. Accordingly, it isconfirmed that on the surfaces of the inorganic powder particles shownin FIG. 3B, even if the inner layer 21 is present, uncovered areas arepresent.

Incidentally, a qualitative analysis was performed for the kneadedmaterial obtained in Example 4 subjected to the fuming nitric acidtreatment by a Fourier transform infrared spectrophotometer (FT-IR). Asa result, a spectrum showing characteristics derived from bondscontained mainly in the component B was obtained.

From the results, it is confirmed that in each Example, the inner layer21 and the outer layer 22 are reliably formed.

3. Evaluation of Sintered Compact

3.1 Evaluation of Sintering Density

The density of each of the sintered compacts obtained in Examples andComparative Examples was measured by a method according to theArchimedean method (specified in JIS Z 2501). Further, from the measuredsintering density and the true density of the inorganic powder, therelative density of the sintered compact was calculated.

3.2 Evaluation of Appearance

The appearance was evaluated according to the following evaluationcriteria by observing 100 sintered compacts obtained in each of Examplesand Comparative Examples.

Evaluation Criteria for Appearance

A: The number of sintered compacts in which cracking, chipping, ordeformation occurred is 3 or less.

B: The number of sintered compacts in which cracking, chipping, ordeformation occurred is 4 or more and 10 or less.

C: The number of sintered compacts in which cracking, chipping, ordeformation occurred is 11 or more and 50 or less.

D: The number of sintered compacts in which cracking, chipping, ordeformation occurred is 51 or more.

3.3 Evaluation of Dimensional Accuracy

The diameters of 100 sintered compacts obtained in each of Examples andComparative Examples were measured by a micrometer. Then, for themeasured values, evaluation was performed according to the followingevaluation criteria based on the “Permissible Deviations in DimensionsWithout Tolerance Indication for Widths” specified in JIS B 0411(Permissible Deviations in Dimensions Without Tolerance Indication forMetallic Sintered Products).

Evaluation Criteria for Dimensional Accuracy

A: Grade is fine (tolerance is ±0.05 mm or less)

B: Grade is medium (tolerance exceeds ±0.05 mm but is ±0.1 mm or less)

C: Grade is coarse (tolerance exceeds ±0.1 mm but is ±0.2 mm or less)

D: Outside the permissible tolerance

The evaluation results of the items 2 and 3 are shown in Tables 2 to 5.

As apparent from Tables 2 to 5, it was confirmed that the respectivesintered compacts obtained in Examples have a higher sintering densitythan the respective sintered compacts obtained in Comparative Examples.Further, it was confirmed that the respective sintered compacts obtainedin Examples have superior appearance and dimensional accuracy to therespective sintered compacts obtained in Comparative Examples.

4. Evaluation of Sample for Evaluation

4.1 Production of Sample for Evaluation

First, in order to clarify the relationship between the grindingconditions and the state of the kneaded material, by using a binderpowder and an inorganic powder, each of which was ground under thefollowing grinding conditions, a kneaded material as a sample forevaluation was produced. As the inorganic powder and the binder powder,the same powders as in Example 3 were used, and kneading was performedunder the same conditions as in Example 3, whereby the kneaded materialwas obtained.

4.2 Evaluation of Viscosity of Sample for Evaluation

Subsequently, the thus produced sample for evaluation was maintained ata temperature of 190° C., and the viscosity thereof was measured using acapirograph. Then, the viscosity was evaluated according to thefollowing evaluation criteria.

Evaluation Criteria for Viscosity

A: The viscosity is within a range in which both the moldability and theshape retainability can be enhanced.

B: The viscosity is within a range in which the shape retainability ishigh but the moldability is slightly poor.

C: The viscosity is within a range in which both the moldability and theshape retainability are poor.

4.3 Evaluation of Sample for Evaluation by Microscopic Observation

Subsequently, the thus produced sample for evaluation was subjected tothe above-described fuming nitric acid treatment, and the outer layer 22was selectively removed from each sample for evaluation.

Then, the remainder was observed by a scanning electron microscope, andan observed image was obtained.

Evaluation Criteria for Microscopically Observed Image

A: A lot of necks are observed (necks are present in 70% or more of theinterspaces between particulate substances).

B: A few necks are observed (necks are present in 20% or more and lessthan 70% of the interspaces between particulate substances).

C: Necks are not observed (necks are present in less than 20% of theinterspaces between particulate substances).

The evaluation results of the items 4.2 and 4.3 are shown in Table 6.Incidentally, the term “neck” refers to a substance which is present soas to connect particulate substances to each other.

TABLE 6 Material Evaluation temperature Grinding machine Grindingmachine Average particle Evaluation results of when grinding temperaturerotation speed diameter results of microscopically [° C.] [° C.] [rpm][μm] viscosity observed image Sample 1 −196 −20 3900 67 B B Sample 2−196 −15 5200 53 A A Sample 3 20 22 8000 55 C C

As apparent from Table 6, it was confirmed that the samples 1 and 2using a powder obtained by cryogenic grinding as the binder powder eachhad a viscosity suitable for shape retainability, and also in thesamples 1 and 2, an inner layer which covers the inorganic powderparticle was formed.

On the other hand, the sample 3 using a powder obtained by grinding atnormal temperature as the binder powder had low shape retainability, andwhen the microscopic observation was performed, an inner layer whichcovers the inorganic powder particle was not observed.

From the above results, it was confirmed that by using a binder powderobtained by cryogenic grinding and also by optimizing the grindingmachine rotation speed and the average particle diameter, a compositionfor injection molding capable of forming a molded body having highershape retainability can be produced.

The entire disclosure of Japanese Patent Application No. 2011-262956filed Nov. 30, 2011 is expressly incorporated by reference herein.

What is claimed is:
 1. A composition for injection molding, comprising:an inorganic powder composed of at least one of a metal material and aceramic material; and a binder containing a polyacetal-based resin andan ethylene-glycidyl methacrylate-based copolymer, wherein theethylene-glycidyl methacrylate-based copolymer is contained in an amountof 1% by mass or more and 30% by mass or less with respect to thepolyacetal-based resin; and the polyacetal-based resin is a copolymer offormaldehyde and a comonomer other than formaldehyde.
 2. The compositionfor injection molding according to claim 1, wherein the composition has:an inner layer composed mainly of the ethylene-glycidylmethacrylate-based copolymer and covering each particle of the inorganicpowder, and an outer layer composed mainly of the polyacetal-based resinand located outside the inner layer.
 3. The composition for injectionmolding according to claim 1, wherein the ethylene-glycidylmethacrylate-based copolymer contains at least one of vinyl acetate andmethyl acrylate as a monomer constituting the copolymer.
 4. Thecomposition for injection molding according to claim 1, wherein thesoftening point of the ethylene-glycidyl methacrylate-based copolymer is65° C. or higher and 105° C. or lower.
 5. The composition for injectionmolding according to claim 1, further comprising a saturated fatty acid.6. The composition for injection molding according to claim 1, furthercomprising a wax.
 7. A sintered compact, which is produced using thecomposition for injection molding according to claim
 1. 8. A compositionfor injection molding, comprising: an inorganic powder composed of atleast one of a metal material and a ceramic material; and a bindercontaining a polyacetal-based resin and an ethylene-glycidylmethacrylate-based copolymer, wherein the ethylene-glycidylmethacrylate-based copolymer is contained in an amount of 1% by mass ormore and 30% by mass or less with respect to the polyacetal-based resin;and wherein the composition has an inner layer composed mainly of theethylene-glycidyl methacrylate-based copolymer and covering eachparticle of the inorganic powder, and an outer layer composed mainly ofthe polyacetal-based resin and located outside the inner layer.
 9. Thecomposition for injection molding according to claim 8, wherein thepolyacetal-based resin is a copolymer of formaldehyde and a comonomerother than formaldehyde.
 10. The composition for injection moldingaccording to claim 8, wherein the ethylene-glycidyl methacrylate-basedcopolymer contains at least one of vinyl acetate and methyl acrylate asa monomer constituting the copolymer.
 11. The composition for injectionmolding according to claim 8, wherein the softening point of theethylene-glycidyl methacrylate-based copolymer is 65° C. or higher and105° C. or lower.
 12. The composition for injection molding according toclaim 8, further comprising a saturated fatty acid.
 13. The compositionfor injection molding according to claim 8, further comprising a wax.14. A composition for injection molding, comprising: an inorganic powdercomposed of at least one of a metal material and a ceramic material; anda binder containing a polyacetal-based resin and an ethylene-glycidylmethacrylate-based copolymer, wherein the ethylene-glycidylmethacrylate-based copolymer is contained in an amount of 1% by mass ormore and 30% by mass or less with respect to the polyacetal-based resin;and wherein the ethylene-glycidyl methacrylate-based copolymer containsat least one of vinyl acetate and methyl acrylate as a monomerconstituting the copolymer.
 15. The composition for injection moldingaccording to claim 14, wherein the composition has: an inner layercomposed mainly of the ethylene-glycidyl methacrylate-based copolymerand covering each particle of the inorganic powder, and an outer layercomposed mainly of the polyacetal-based resin and located outside theinner layer.
 16. The composition for injection molding according toclaim 14, wherein the polyacetal-based resin is a copolymer offormaldehyde and a comonomer other than formaldehyde.
 17. Thecomposition for injection molding according to claim 14, wherein thesoftening point of the ethylene-glycidyl methacrylate-based copolymer is65° C. or higher and 105° C. or lower.
 18. The composition for injectionmolding according to claim 14, further comprising a saturated fattyacid.
 19. The composition for injection molding according to claim 14,further comprising a wax.